This invention relates in general to surface verification and correction in the process of manufacturing, and more specifically, to a method and system for producing an optical, topographical display of surface profile errors directly on the surface.
In the process of manufacturing a large part, such as a tool, a mold, or a stamped sheet metal panel where it is important that the surface be precisely shaped, e.g. as in the aerospace industry, it is usually required to verify and, if necessary, to accurately correct (rework) the shape of the surface. Obviously, the surface shape has to be compared with design specifications. In modern production, parts, tools, dies, and molds are usually designed based on computer-assisted design (CAD) models. In computer-aided manufacturing (CAM, for example, computer numerically-controlled (CNC) machine tools use the CAD design as an input to control the operation of the tool in machining a product.
There are a number of methods for precise verification of a surface shape using different profilometers, coordinate measuring machines (CMM's), laser trackers, 3D scanning measurement systems and other equipment.
Some manufacturers such as FARO Technologies, Inc. (“FARO”) make manual CMM's that use an articulated arm with six degrees and a contact sensor attached to a free end of the arm. The arm is being moved over the surface by an operator in the manner of a pantograph. Transducers at the articulations supply angular positions which can be converted into surface profile information. Other manufacturers such as Brown and Sharp make automated CMM's based on multi-axis stages and gantries carrying contact or non-contact sensors capable to gather information about a surface profile. Conventional CMM's have limited usage in large scale manufacturing, for example, in aerospace or boat building industries. Most appropriate for large surface profile measurements in those industries are non-contact 3D scanning systems and laser trackers. The precision required for tool surface profiling in aerospace applications is quite high—usually several thousandths of an inch. Non-contact 3D scan measurement system capable of delivering such precision is manufactured by MetricVision. It is suitable for automated surface measurement, but this system is very expensive. Laser trackers generally provide the same high level of precision but they are less expensive and widely used in large scale industrial applications despite substantial manual labor involved in a surface scan.
High precision laser trackers are manufactured by Leica Geosystems AG (“Leica”), Automated Precision, Inc. (“API”), and FARO. Known laser trackers are single point devices. The tracker measures any surface point location by directing a laser beam toward a remote optical probe that touches the surface at a given point. The optical probe made as a ball, usually 0.5-1 inch in diameter, that has a very precise retro-reflective prism inside. The retro-reflective prism is facing away from the surface being measured and it returns the incident laser beam back to its source, which allows tracker's angular servo system to capture the direction of the beam and to follow (track) movement of the ball. Probes of this type are commonly termed a “pick-up ball”. When acquiring the surface profile the pick-up ball is being moved by hand from point to point while keeping it in touch with the surface. This process, with periodic recordation of position information, creates so-called “cloud” of digitized surface data points.
Usually trackers or other surface scan measurement systems utilize integrated software capable of comparing measurement data with CAD model data, computing deviations between the actual surface shape and its model, and presenting results in graphic form on a computer screen or a plotter. It helps to analyze surface imperfections and to decide if the tested part is within specifications or not. Some of parts and, especially, large tools in aerospace industry are so expensive in production that it is often more reasonable to manually rework and correct their surface flaws rather to completely remake them. However, for the surface rework process it is not enough to see detected imperfections on a computer screen but it is absolutely necessary to map and locate those imperfections directly on the tested surface.
In today's industrial practice this mapping of the imperfections on the tested surface is done manually, literally with a ruler and a pencil, by drawing auxiliary lines on the surface and marking surface errors point-by-point. It is a very difficult and time-consuming process. For large precise parts it can take weeks of time to complete. And every time when a particular area on the surface is corrected by filling or grinding, all marks are being erased. So, if after second verification it is necessary to “touch up” or additionally correct that area, the manual mapping and marking has to be done again. Manual error mapping also includes a quite difficult procedure to reference the region of rework with respect to datum features of a part. That sometimes requires one to map out and mark an auxiliary mesh grid lined onto the surface.
U.S. Pat. No. 6,365,221 to Morton describes a computer controlled method for fairing and painting marine vessel surfaces. Morton uses multiple robots positioned on moveable transports. Arms have various attachments such as laser surface mapping systems, compound and paint sprayers, and milling and vacuum apparatus. No projection of information about surface variations onto the vessel is described, nor is there any disclosure of a comparison of the actual surface to a design surface. This system is limited to applications, such as marine vessel manufacturing and refurbishing, where a relatively coarse level of precision with respect to the surface, e.g. about ⅛ of an inch, is acceptable. This is an expensive system that is economically reasonable only for processing huge surfaces. In contrast, aerospace manufacturing processes, e.g., typically require 10 times better precision.
Thus there is a need to improve, accelerate, and reduce cost of the existing processes of manual surface correction in aerospace and other industries that demand a high degree of precision.
It is therefore a principal object of this invention to provide a method and system for visualizing errors of a surface shape by optically projecting onto the actual surface a topographical map of deviations of that surface from a nominal, design surface.
Another object of the invention is to continuously display on a surface a mapping in patterns (contours or areas) of the same or generally the same deviation over all, or significant portions of, the surface.
Another object of the invention is to reduce the time required to rework a surface, particularly a large surface such as on a mold, die, tool, or formed panel of a product such as an airplane fuselage.
Still another object is to provide such an on-surface projection without requiring an auxiliary mesh grid to be mapped onto the surface.
A further object is to provide a system that is comparatively compact, mobile, and readily removed from and repositioned with respect to the surface being tested and/or reworked.